1. Field of the Invention
The present invention relates generally to liquid junction structures for electrochemical electrodes and, more particularly, to shrink tube liquid junction structures useful in electrochemical combination and reference electrodes.
2. Description of the Prior Art
Electrochemical ion measurements are typically made by contacting a test solution with a measuring electrode and a reference electrode and measuring the potential difference between the two electrodes. The measuring electrode is typically a high impedance device for measuring activity or concentration of the ion in question and accordingly generates a potential with respect to the test solution related to the concentration of the ion of interest. The reference electrode provides a potential that is substantially independent of the test solution composition. Typically, the reference electrode comprises a metal conductor in contact with a mass of sparingly soluble salt of the metal and immersed in an electrolyte having a nonmetallic ion in common with the metal salt. Such reference electrodes often employ a silver wire conductor coated at one end with silver chloride and immersed in a potassium chloride electrolyte solution contained within the electrode.
It is now commonplace for a reference electrode and a measuring electrode to be combined in a single structure referred to as a combination electrode having an ion measuring portion and a reference portion. In a typical form a combination electrode comprises an inner generally tubular electrode body having an ion measuring structure at one end, a generally tubular container surrounding and receiving the inner electrode body, and a reference electrolyte reservoir defined in the annular space between the body and container.
To make ion measurements, it is necessary for the reference electrolyte, within a reference electrode or within the reference portion of a combination electrode, to electrolytically contact the test solution to be measured. For this purpose numerous so-called liquid junction structures have been developed to establish an electrolytic path between the electrolyte and the test solution. Typically such is accomplished by means of a wettable capillary material which provides either a minute flow rate path or an ion diffusion path between the electrolyte and the test solution. An early but still popular liquid junction structure employs one or more strands of capillary fibers, such as asbestos or linen, in contact with the reference electrolyte and with the test solution. U.S. Pat. No. 4,012,308 illustrates one such liquid junction comprising a plurality of asbestos fiber strands around and extending through an annular seal between an inner glass pH electrode body and an outer tubular plastic container. The strands are looped through an elastomeric band which surrounds the glass electrode body and holds the strands in place against the glass body.
In another fiber strand liquid junction a plastic shrink tube positioned about and shrunk onto the inner glass pH electrode body traps and restrains the asbestos fiber strands in place against the glass electrode body.
While satisfactory electrodes result with the two foregoing liquid junction structure arrangements, their operation is compromised by the existence of undesirable stirring artifacts and liquid junction potentials. Such result because an ideal liquid junction structure through the asbestos fibers alone is not attained in actual practice. It is believed that since the asbestos strands are restrained by the shrink tubing against the relatively hard outer surface of the inner glass electrode body, voids or pockets remain along the length of the fiber material adjacent the hard glass body in which secondary parallel paths bypassing the fiber material develop between the reference electrolyte and the solution to be tested. Such result in the aforementioned stirring artifacts and liquid junction potentials which reduce the accuracy and reliability of the electrode.